QVLA: add a Q_ASSERT

GCC's -fanalyzer complains that `ptr` might be null. It's actually
never null (by class invariant).

Change-Id: I5660b1e766b2cfde3f042b8e87168987e7618591
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
Reviewed-by: Andrei Golubev <andrei.golubev@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qvarlengtharray.h

@@ -492,6 +492,7 @@ template <class T, qsizetype Prealloc>
 Q_OUTOFLINE_TEMPLATE void QVarLengthArray<T, Prealloc>::reallocate(qsizetype asize, qsizetype aalloc)
 {
     Q_ASSERT(aalloc >= asize);
+    Q_ASSERT(ptr);
     T *oldPtr = ptr;
     qsizetype osize = s;